Processes for converting lower oxygenates such as methanol to hydrocarbons are known and have become of great interest in recent times because they offer an attractive way of producing liquid hydrocarbon fuels, especially gasoline, from sources which are not of liquid petroleum origin. In particular, they provide a way by which methanol can be converted to gasoline boiling range products in good yields. The methanol, in turn, may be readily obtained from coal by gasification to synthesis gas and conversion of the synthesis gas to methanol by well-established industrial processes. As an alternative, the methanol may be obtained from natural gas by other conventional processes, such as steam reforming or partial oxidation to make the intermediate syngas. Crude methanol from such processes usually contain a significant amount of water, usually in the range of 4 to 20 wt %.
The conversion of methanol and other lower aliphatic oxygenates to hydrocarbon products may take place in a fixed bed process as described in U.S. Pat. Nos. 3,998,899; 3,931,349 and 4,035,430. Fluidized bed catalysis has been described in U.S. Pat. Nos. 4,251,484 (Daviduk et al) and 4,513,160 (Avidan and Kam). In the fixed bed process, the methanol is usually first subjected to a dehydrating step, using a catalyst such as gamma-alumina, to form an equilibrium mixture of methanol, dimethyl ether (DME) and water. This mixture is then passed at elevated temperature and pressure over a catalyst such as ZSM-5 zeolite for conversion to the hydrocarbon products which are mainly in the range of light gas to gasoline. Water may be removed from the methanol dehydration products prior to further conversion to hydrocarbons and the methanol can be recycled to the dehydration step, as described in U.S. Pat. No. 4,035,430. Removal of the water is desirable because the catalyst may tend to become deactivated by the presence of excess water vapor at the reaction temperatures employed; but this step is not essential.
In the operation of an adiabatic fixed bed process, a major problem is thermal balance. The conversion of the oxygenated feed stream (methanol, DME) to the hydrocarbons is a strongly exothermic reaction liberating approximately 1480 kJ. (1400 Btu) of heat per kilogram of methanol. In an uncontrolled adiabatic reactor this would result in a temperature rise which would lead to extremely fast catalyst aging rates or even to damage to the catalyst. Furthermore, the high temperatures which might occur could cause undesirable products to be produced or the product distribution could be unfavorably changed. It is therefore necessary that some method should be provided to maintain the catalyst bed within desired temperature limits by dissipating the heat of the reaction.
One method is to employ a light gas portion of the hydrocarbon product as recycle, as described in U.S. Pat. No. 3,931,349 (Kuo). Typically, cooled light hydrocarbon gas, rich in methane, ethane, etc., is separated from the C.sub.5.sup.+ gasoline and C.sub.3 -C.sub.4 LPG products, re-compressed and reheated before being mixed with the reactant feedstream entering the bed of conversion catalyst. Although effective in controlling bed temperature, the expense of cooling the recycle gas, compressing it and re-heating it add to the cost of the conversion.
Typically the crude methanol feedstock employed in MTG processes contains about 4 to 20 wt % water as the principal impurity. Excessive water not only contributes to catalyst deactivation, but also requires larger volume equipment to handle the increased throughput. Various proposals have been put forth for reducing the water content of crude methanol, for instance the distillation system described by Mao et al in copending U.S. patent application Ser. No. 823,153, filed Jan. 27, 1986, incorporated by reference.
It is main object of the present invention to provide a novel and economic technique for removing excess water from crude MTG feedstocks, including novel operating methods and equipment for treating these oxygenate feedstocks.